This invention relates to a method for providing an improved block copolymer from .alpha.-methylstyrene, an alkylstyrene comonomer and a diorganocyclotrisiloxane. In one aspect this invention relates to thermoplastic block copolymers of the type (BAB).sub.n wherein A denotes a thermoplastic, predominantly poly-.alpha.-methylstyrene block, which is coupled to the B blocks by way of an alkylstyrene comonomer, and B denotes an elastomeric polydiorganosiloxane block, said block copolymer having an upper operating temperature limit which is greater than that of the .alpha.-methylstyrene-diorganosiloxane block copolymers of the art.
Block copolymers comprising polyvinylmonomer segments, such as polystyrene, and polydiorganosiloxane segments are known from Morton, et al., U.S. Pat. No. 3,051,684, Bostick, U.S. Pat. No. 3,483,270, and Saam, et al. U.S. Pat. No. 3,873,636. Block copolymers of poly-.alpha.-methylstyrene and polydiorganosiloxane wherein the poly-.alpha.-methylstyrene blocks are not bonded directly to the polydiorganosiloxane blocks, but rather, are bonded indirectly through a styrene comonomer, are disclosed by Saam, et al., U.S. Pat. Nos. 3,665,052; 3,678,125, and 3,678,126 and by Kendrick, et al. U.S. Pat. No. 3,890,405. Similar block copolymers are disclosed by Davies, et al., Canadian Pat. No. 983,194 wherein styrene is preferred as the vinylic comonomer but other organic vinylic monomers are allowed if they are capable of being anionically polymerized and if their anion reacts with the diorganocyclotrisiloxane. Block copolymers of poly-.alpha.-methylstyrene and polydiorganosiloxane, which are prepared by anionic polymerization, need this coupling unit between the two blocks to stabilize the poly-.alpha.-methylstyrene block against depolymerization under the conditions that are used when the poly-.alpha.-methylstyrene is reacted with the diorganosiloxane monomer, i.e. hexaorganocyclotrisiloxane.
The poly-.alpha.-methylstyrene-polydiorganosiloxane block copolymers of the art, wherein the poly-.alpha.-methylstyrene block is bonded to a polydimethylsiloxane block by way of a short polystyrene segment, have an upper glass transition temperature, hereinafter Tg, of approximately 125.degree. C. This value is unexpectedly low because the upper Tg of a poly-.alpha.-methylstyrene-polydiorganosiloxane block copolymer is expected to be approximately equal to the Tg of the poly-.alpha.-methylstyrene block, which has a value of approximately 155.degree. C. at M.sub.A of 8,000 and a value as high as 180.degree. C. at higher molecular weights. A second Tg, characteristic of the polydiorganosiloxane block is also expected; however, the Tg of most polydiorganosiloxanes is less than the Tg of poly-.alpha.-methylstyrene and does not constitute an upper operating temperature limit for a poly-.alpha.-methylstyrene-polydiorganosiloxane block coploymer.
Thermoplastic block coploymers derive their physical properties, such as tensile strength, in part from the strength of their glassy thermoplastic block. Consequently, considerable strength is lost when said block copolymers are subjected to temperatures above their Tg; the Tg thereby constituting an upper operating temperature limit for the block copolymer for many uses. It is therefore desirable to increase the Tg of poly-.alpha.-methylstyrene-polydiorganosiloxane block copolymers, thereby providing block copolymers having an increased upper operating temperature limit and hence being suitable for use in more stringent thermal environments.
Although the references cited above suggest that certain alkylstyrenes may be used to prepare polyvinylmonomer-polydiorganosiloxane block copolymers, there is no suggestion that a poly-.alpha.-methylstyrene block can be coupled to a polydiorganosiloxane block by way of an alkylstyrene segment to prepare a block copolymer, nor is there any suggestion that such a block copolymer would have improved properties.